1. Field of the Invention
The present invention relates to a toner for electrophotography. More particularly, the invention relates to a toner for electrophotography having the required physical properties when thermally fixing an unfixed toner layer to obtain a stable image during printing.
2. Description of the Related Art
In an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, an electrostatic latent image is formed through light-exposure on a photoreceptor which is uniformly charged. A toner is attached to the electrostatic latent image, and a resulting toner image is transferred to a transfer medium such as a sheet of paper. Then, an unfixed toner image is fixed on the transfer medium through several processes such as heating, pressing, solvent steaming, and so on. In most fixing processes, the transfer medium with the toner image passes through fixing rollers and pressing rollers, and by heating and pressing, the toner image is fused on the transfer medium. Toner is fixed on the transfer medium according to fixing conditions to form a stable image.
In a thermal fixing method in which a heat roller or film is used, the surface of the heat roller or film contacts the toner image on a fixing sheet. In this case, a high heat efficiency is required to melt and attach the toner image on the fixing sheet. In the thermal fixing method, it is necessary to increase the heat capacity of a heat fixing means to avoid a fixing failure caused by passage of the fixing sheet and fixing at a low temperature. To realize low power consumption while retaining the fixing property of a toner, toner performances, particularly the fixing property of toner at a low temperature should be improved.
For example, in a fixing step of a pressure-thermal fixing method, hot rollers, under pressure, contact a toner image which is in a melted state. Thus, some of the toner is transferred from the paper and is attached to the surface of the fixing roller, and then transferred to a fixing sheet, resulting in contamination of the fixing sheet. This is called as “offset” and is significantly affected by fixing rate and temperature. Generally, if the temperature of the fixing roller surface is low then the fixing rate is slow and if the temperature of the fixing roller surface is high then the fixing rate is fast. This is because a fixed amount of energy is supplied to a toner image to fix the toner image regardless of fixing rates.
To solve such problems, the fixing temperature is generally increased to facilitate fixing a toner on a fixing sheet to ensure the fixing rate is fast. According to this method, the temperature of heat rollers can be somewhat lowered to avoid offset of a top toner layer at a high temperature. However, when very high shear force is applied to the toner layer, problems such as winding offset in which a fixing sheet winds around fixing rollers and marks from separating means for separating the fixing sheet from the rollers generated in a fixed image, etc. are caused.
Korean Patent No. 138,583, Korean Patent Laid-Open Publication No. 2001-083034, and Korean Patent Laid-Open Publication No. 1999-063467 disclose toners for electrophotography having specific rheological properties. However, they fail to obtain both of superior fixing property and anti-offset property. Thus, there is need for a technology that can predict the behavior of a toner under heat and pressure to improve a fixing property and prevent offset.